The combustion of gasoline in reciprocal engines requires, as it is well known, a flame initiation device commonly called an ignition system. An ignition systems consists of two main components:                a spark plug; and        an ignition coil or transformer.        
The spark plug represents the direct interface to the flame kernel itself via its firing face and represents an isolated electrical feed-through into the combustion chamber. The task of the ignition transformer is to provide the suitably shaped energy to initiate the combustion. This is conventionally split into two consecutive and distinct phases.
The first phase stores electrical energy inside the inductors of the transformer and the next phase releases the previous stored energy. The transition itself creates a sufficient over-voltage at the spark-plug firing face, which allows initiating a dielectric break down and thereby changes significantly the electrical properties of the load of such electrical network. Because of the change in load the remaining stored energy undergoes depletion into the dielectric break down providing the spark. This ultimately creates the desired shockwave, radicals and heat and thereby, if well surrounded by combustible gasoline mixtures, a flame kernel, which in consequence will initiate the combustion.
For operating with lean gasoline mixtures, the common ignition systems fail (or limit the lean operation) because of the typical discharge nature of the stored energy to the load interaction. The depletion of the remaining stored energy of the transformer into the spark, which itself interacts heavily with its surroundings in the combustion chamber, creates unpredictable load situations. Accordingly, unpredictable heat amounts are delivered, in particular at unfavorable timings and unexpected locations. This consequently tends to result in statistical scattering of the combustion pressure, which contributes to unfavorable engine-out emissions as well as uncontrollability also referred to as instability of the combustion.
To a certain extent this malfunction is caused by the depletion of the energy of the transformer, thus the collapsing of the delivered electrical power into the spark.
The conventional solution to this is to simply increase the amount of energy stored in the transformer. Many higher energy coils are on the market and help solving the problem.
Other technical solutions are multi-charge ignition (MCI) systems. MCI systems are simply based on multiple repetitions of the aforementioned two consecutive distinct phases. A transformer comprises one primary winding magnetically coupled to one secondary winding. For one combustion event, the primary winding is repetitively energized and disenergized to create the series of sparks. These systems deliver over time several individual sparks in respect of one combustion event of a combustion cycle. The advantage is that more heat is disposed over a longer time, but not continuously. There are still combustion events when no spark-heat occurs while most suitable combustible mixtures are present. This is leading occasionally to very timely tight stable combustion situations, were smallest disturbances create increased pressure scatter traces and thereby lead to unstable lean operation conditions.
EP 2 325 476 discloses a multi-charge ignition system comprising two transformers that are operated alternately to maintain a burn phase.
EP 2 141 352 describes an ignition system with a dual primary coil, wherein the primary windings are alternately energized and deenergized, the first primary winding being reenergized whilst the second primary winding is deenergized, etc., whereby it is possible to successively cycle between an arc generated by the first primary winding and an arc generated by the second primary winding. A practical problem of this system is however the alternating polarities of the current in the secondary winding, which prevents the use of a diode in the line leading from the secondary winding terminal to the spark plug. Absent such diode, it is not possible to prevent a so-called “early make” spark, which typically occurs at the moment the primary coil is switched to the power source to start the charging phase. The occurrence of early make spark triggers ignition at undesired timings at low engine pressure.
U.S. Pat. No. 3,280,809 describes an ignition system of complex design, featuring a transformer having 3 primary windings and 1 secondary winding. The burn phase is maintained by alternating between two primary windings, and an alternating output current is produced.